mmc: am654_sdhci: Separate J721E compatible into 8bit and 4bit versions

The j721e 4 bit instances don't have a hard DLL and therefore don't need
any DLL related configurations. Split the compatibles into an 8 bit and a
4 bit one. Add a private flags field which can be used to check if the
DLL is present and don't register the set_ios_post callback for the 4 bit
compatible instances.

Also update the compatibles in k3-j721e-main.dtsi to avoid breaking boot
with the new compatibles.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>